1. Field of the Invention
The present invention relates to a developing device for use in a copier, facsimile apparatus, printer or similar image forming apparatus, an image forming apparatus using the same and a process cartridge.
2. Description of the Background Art
A developing device of the type using only toner grains, i.e., a one-component type of toner not containing carrier grains is extensively used. This type of developing device is configured to charge the toner grains in a casing by agitating them and then convey the charged toner grains to a toner carrier. The toner grains thus deposited on the toner carrier are regulated in amount by a metering member and then brought to a developing zone where the toner carrier and an image carrier face each other at the shortest distance. At the developing zone, the toner grains are transferred from the toner carrier to the image carrier to thereby develop a latent image formed on the image carrier.
While the toner grains deposited on the toner carrier should ideally be entirely charged to preselected polarity, some of such toner grains are, in practice, not fully charged. Should the toner grains with short charge be conveyed via the metering member, they would fly away from the toner carrier and would thereby smear the inside of the image forming apparatus as well as paper sheets or similar recording medium.
Further, toner and members that strongly rub against the toner, e.g., the toner carrier, a toner feeding member and the metering member vary with the elapse of time, and so does the toner electrifying ability. As a result, it soon becomes difficult for such members to electrify each other. In addition, it is likely that additives present on the surfaces of the individual toner grains are separated from or buried in the surfaces of the toner grains and fail to exhibit a fluidity enhancing function and an electrification control function expected thereof. Consequently, the ratio of toner grains with short charge to the entire toner grains is apt to increase, and toner grains of opposite polarity and short charge are also apt to increase. If such toner grains are conveyed to the developing zone by the toner carrier, then they smear the background of an image and thereby lower the quality of the image.
In light of the above, various devices have heretofore been proposed to control stress to act on toner grains or to discharge a developer from a developing device before the developer is critically deteriorated. Japanese Patent Laid-Open Publication No. 2002-333764, for example, proposes to obviate, e.g., the blow-off or the blocking of toner grains ascribable to the excessive replenishment from a toner replenishing or storing section to a developing section by returning excessive part of the toner grains to the toner replenishing section. At this instant, part of toner grains with short charge is also returned to the toner replenishing section because of the replacement of toner grains, so that image quality is maintained at a certain acceptable level.
However, the system proposed by the above document has some problems left unsolved. For example, toner grains replenished from the toner replenishing section are soon returned to the replenishing section or not smoothly returned to the replenishing section with the result that excess toner grains increase dynamic torque to an unusual level.